Method and apparatus to control output power from a switching power supply

ABSTRACT

Techniques are disclosed to control output power from a switching power supply. A sample power converter controller circuit includes a control circuit coupled to be coupled to a switch. The control circuit includes a current sense circuit to be coupled to a primary winding of an energy transfer element coupled to the switch. The control circuit also includes a measurement circuit to be coupled to a secondary winding of the energy transfer element. A feedback circuit is also included and is coupled to the control circuit and coupled to receive a feedback-signal derived from an output of a power converter. The feedback circuit is coupled to output a signal to the measurement circuit. An output of the measurement circuit and an output of the current sense circuit are coupled to control a switching of the switch to regulate a combination of an output voltage and an output current of the power converter. A combination of the output voltage and the output current corresponds to at least one of a regulated output region and an unregulated output region. At least one of the unregulated output regions is a self protection auto-restart region. Within at least one unregulated output region the power converter controller circuit provides continuous output power at a substantially maximum output power of the power converter. Each output region corresponds to a magnitude and duration of the feedback signal.

REFERENCE TO PRIOR APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/038,625, filed Jan. 18, 2005, now U.S. Pat. No. 7,272,025.

BACKGROUND

1. Technical Field

The present invention relates generally to electronic circuits, and morespecifically, the invention relates to switching power supplies.

2. Background Information

A common application of switching power supplies is to charge batteries.The output power of a battery charger is usually controlled to provide aregulated voltage and a regulated current. The voltage is regulatedbetween a maximum and a minimum voltage over a range of output current.The current is regulated between a maximum and a minimum current over arange of output voltage. As the battery charges, there is usually mabrupt transition from regulated output current to regulated outputvoltage that occurs automatically when the battery voltage reaches athreshold. That is, the locus of output voltage and output currentplotted in Cartesian coordinates usually has a sharp corner at the pointof transition that corresponds to the point of maximum output power.Typically, there is also a requirement to substantially reduce theoutput current when the voltage falls below a threshold to preventdamage from a short circuit or similar fault on the output.

The practice of designing a battery charger to have a sharp transitionbetween regulated voltage and regulated current can result in a productthat costs more than necessary to provide the desired function. It isoften possible to reduce the cost of the batten charger and to meet allrequirements by designing an unregulated transition between theregulated voltage and the regulated current. The output voltage andoutput current in the region of unregulated transition is bounded by thenatural output characteristics of the switching regulator, and typicallyfollows the curve of maximum output power for a given output voltage orcurrent.

To achieve lower cost, the switching regulator is designed to operatewith a control circuit that permits the regulator to make an unregulatedtransition between regulated output voltage and regulated output currentsuch that the voltage and current are maintained within the specifiedboundaries. Proper design of the unregulated transition within thespecified boundaries reduces the maximum power output, allowing the useof components that are less costly than the components to guaranteehigher output power. The control circuit operates the switchingregulator for regulated voltage, regulated current, unregulatedtransition, or self-protection according to the magnitude of a feedbacksignal.

Battery chargers typically use one circuit to sense the output voltageand a different circuit to sense the output current for the purposes ofregulation. In many applications, it is possible to eliminate thecircuitry that senses output current, and to use an unregulatedtransition between the regulated output voltage and self-protectionthreshold voltage to satisfy the requirements of the design. Eliminationof circuitry to sense output current reduces cost and raises efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention detailed illustrated by way of example and notlimitation in the accompanying Figures.

FIG. 1 is a functional block diagram of one embodiment of a switchingregulator that may control output power in accordance with the teachingof the present invention.

FIG. 2 is a diagram that shows the boundaries of output voltage andoutput current of one embodiment of a switching regulator that controlsoutput power in accordance with the teachings of the present invention.

FIG. 3 shows four specific regions of operation within the boundaries ofoutput voltage and output current for one embodiment of a switchingregulator that controls output power in accordance with the teachings ofthe present invention.

FIG. 4 illustrates three specific regions of operation within theboundaries of output voltage and output current for another embodimentof a switching regulator that controls output power in accordance withthe teachings of the present invention.

FIG. 5 is a flowchart of one embodiment of a method to control outputpower for an embodiment of a switching regulator in accordance with theteachings of the present invention.

FIG. 6 describes the control characteristics of one embodiment of aswitching regulator that uses pulse width modulation to regulate anoutput in accordance with the teachings of the present invention.

FIG. 7 describes the control characteristics of one embodiment of aswitching regulator that uses an on/off control to regulate an output inaccordance with the teachings of the present invention.

FIG. 8 shows the output voltage and current characteristics of a typicalswitching regulator battery charger controlled in accordance with theteachings of the present invention.

FIG. 9A is one embodiment of a circuit diagram of power supply with anintegrated circuit controller in accordance with the teachings of thepresent invention.

FIG. 9B is another embodiment of a circuit diagram of power supply withan integrated circuit controller in accordance with the teachings of thepresent invention.

DETAILED DESCRIPTION

Embodiments of a power supply regulator that may be utilized in a powersupply are disclosed. In the following description, numerous specificdetails are set forth in order to provide a thorough understanding ofthe present invention. It will be apparent, however, to one havingordinary skill in the art that the specific detail need not be employedto practice the present invention. Well-known methods related to theimplementation have not been described in detail in order to avoidobscuring the present invention.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

Techniques are disclosed to provide an unregulated mode of operation fora power supply that allows it to meet the requirements of a batterycharger at a lower cost than conventional solutions. To illustrate, FIG.1 shows a functional block diagram of a power supply that may include anembodiment of a power supply regulator that is a battery charger inaccordance with the teachings of the present invention. The topology ofthe power supply illustrated in FIG. 1 is known as a flyback regulator.It is appreciated that there are many topologies and configurations ofswitching regulators, and that the flyback topology shown in FIG. 1 isprovided to illustrate the principles of an embodiment of the presentinvention that may apply also to other types of topologies in accordancewith the teachings of the present invention.

The power supply in FIG. 1 provides output power to a load 165 from anunregulated input voltage V_(IN) 105. In one embodiment, the load 165may be a rechargeable battery. The input voltage V_(IN) 105 is coupledto an energy transfer element T1 125 and a switch S1 120. In the exampleof FIG. 1, the energy transfer element T1 125 is coupled between aninput of the power supply and an output of the power supply. In theexample of FIG. 1, the energy transfer element T1 125 is illustrated asa transformer with two windings. In general, the transformer can havemore than two windings, with additional windings to provide power toadditional loads, to provide bias voltages, or to sense the voltage at aload. A clamp circuit 110 is coupled to the primary winding of theenergy transfer element T1 125 to control the maximum voltage on theswitch S1 120. Switch S1 120 is switched on and off in response to oneembodiment of a controller circuit 145 in accordance with the teachingsof the present invention. In one embodiment, switch S1 120 is atransistor such as for example a power metal oxide semiconductor fieldeffect transistor (MOSFET). In one embodiment, controller 145 includesintegrated circuits and discrete electrical components. The operation ofswitch S1 120 produces pulsating current in the rectifier D1 130 that isfiltered by capacitor C1 135 to produce a substantially constant outputvoltage V_(O) or output current I_(O) at the load 165.

The output quantity to be regulated is U_(O) 150, that in general couldbe an output voltage V_(O), an output current I_(O), or a combination ofthe two. The regulated quantity is not necessarily constant, but can beregulated to change in a desired way in response to a feedback signal.An output that does not respond to a feedback signal is unregulated. Afeedback circuit 160 is coupled to the output quantity U_(O) 150 toproduce a feedback signal U_(FB) 155 that is an input to the controller145. Another input to the controller 145 is the current sense signal 140that senses a current I_(D) 115 in switch S1 120. Any of the may knownways to measure a switched current, such as for example a currenttransformer, or for example the voltage across a discrete resistor orfor example the voltage across a transistor when the transistor isconducting, may be used to measure current I_(D) 115. FIG. 1 alsoillustrates an example waveform for current I_(D) 115 to show theparameters that the controller can adjust to regulate the outputquantity U_(O) 150. The maximum of current I_(D) 115 is I_(MAX), theswitching period is T_(S), and the duty ratio is D. The controllertypically limits the duty ratio to a maximum D_(MAX) that is less than100%.

In one embodiment, the controller 145 operates switch S1 120 tosubstantially regulate the output U_(O) 150 to its desired value. In oneembodiment, the output U_(O) changes from an output voltage to an outputcurrent in response to the magnitude of the output voltage or the outputcurrent. In one embodiment, controller 145 includes an oscillator thatdefines substantially regular switching period T_(S). In one embodiment,regulation is accomplished by control of the conduction time of theswitch within a switching period. In each switching period, the fractionof the switching period that the switch is closed is the duty ratio D ofthe switch. In one embodiment, regulation is accomplished by control ofthe maximum current I_(MAX) of the switch. In another embodiment,regulation is accomplished by control of the switching period T_(S).

In one embodiment, the parameters of the regulator are substantiallyindependent of the feedback signal U_(FB) over a range of values ofU_(FB). When substantially independent of feedback signal U_(FB),parameters I_(MAX), D, and T_(S) can be either fixed or allowed to varyin response to changes in other quantities such as for example the inputvoltage V_(IN) 105 or the load 165. In one embodiment such changes aredetermined by the natural characteristics of the topology of the powerconverter, such as the flyback topology. Thus, one embodiment of aregulator can be designed so that an output behaves in a desired waywhen it is unregulated by a feedback signal in accordance with theteachings of the present invention.

FIG. 2 shows boundaries for output voltage and output current of oneembodiment of a switching power supply that operates in accordance withthe teachings of the present invention. The output of the power supplyfalls within the outer boundary 205 and the inner boundary 210. Theouter boundary sets a maximum output voltage V_(OMAX) and a maximumoutput current I_(OMAX) that define a maximum output power P_(MAX) atthe intersection 200 of the lines for V_(OMAX) and I_(OMAX). A powersupply that has output characteristics within the region of the solidlines 215 will operate between the outer boundary 205 and the innerboundary 210 at less than the maximum output power P_(MAX). Such a powersupply will typically cost less than one capable of operation atP_(MAX).

FIG. 3 shows four specific regions within the boundaries of outputvoltage and output current for one embodiment of a power supply thatoperates in accordance with the teachings of the present invention. TheCV region 300 is a region of regulated voltage where the variation inoutput voltage is restricted over a wide range of output current. The CCregion 310 is a region of regulated current where the variation inoutput current is restricted over a wide range of output voltage. The UTregion 305 is a transition region where the output voltage and theoutput current arc unregulated between the CV region 300 and the CCregion 310. The AR region 315 is an auto-restart region where the powersupply operates at a reduced output voltage and reduced average outputcurrent to avoid damage from a short circuit fault on the output or froma fault that prevents the feedback signal from reaching the controller.In the AR auto-restart region, the power supply operates in anauto-restart cycle. In one embodiment in the auto-restart cycle, thecontroller allows the power switch to operate unregulated for a durationthat is long enough to raise the output of the power supply above anauto-restart threshold when the load is within specifications, followedby a long interval of no switching if the output does not reach thethreshold during the allowed duration of the switching. The auto-restartcycle repeats until the output of the power supply rises above theauto-restart threshold. It will be understood that not all applicationswill require an auto-restart region of operation. Some applications ofembodiments of the present invention benefit from the UT region 315without the AR region 315.

Some applications of embodiments of the present invention do not requirethe two regions of regulated operation. FIG. 4 illustrates how oneembodiment of a power supply may operate within the specified boundariesof output voltage and output current with only one regulated region,constant voltage CV region 400. An unregulated transition UT region 405is between a regulated constant voltage CV region 400 and anauto-restart AR region 415. In applications that do not require anauto-restart AR region 415 for self-protection, requirements may besatisfied with only a constant voltage CV region 400 and an unregulatedtransition region UT region 405. In one embodiment, the auto-restart ARregion is not required, and the UT region 405 is extended to zero outputvoltage without substantial reduction in output current

FIG. 5 is a flowchart illustration that describes one embodiment of amethod to control the output power of a switching regulator inaccordance with the teachings of the present invention. As shown, afeedback signal U_(FB) is measured on block 503. The magnitude of thesignal is then compared to thresholds in block 505. The results of thecomparisons determined in blocks 507, 509 and 511 are used to select aparticular region of operation. In FIG. 5, the thresholds are U₁, U₂,and U₃ with 0<U₁<U₂≦U₃.

If the feedback signal is sufficiently small (less than or equal to U₁),the controller operates the regulator in the unregulated auto-restart ARregion as indicated in block 513. If the feedback signal is larger thanan upper threshold (greater than or equal to U₃), the controlleroperates the regulator in the unregulated inhibited switching mode sothat the power switch is off as indicated in block 515. For othermagnitudes of the feedback signal, the controller either regulates thepower supply output in regulated mode, as indicated in block 519 oroperates the regulator in an unregulated mode to produce a maximumunregulated power, as indicated in block 517.

In one embodiment, the change in operation between regions is notinstantaneous when the feedback signal crosses a feedback threshold. Inone embodiment, the value of the feedback signal satisfies a thresholdcondition for an established time before the controller will change theoperation to a different region in accordance with the teachings of thepresent invention.

The power supply responds to the magnitude of the feedback signal U_(BF)to operate in one of the output regions. In one embodiment, themagnitude of the feedback signal U_(FB) determines the operation of thepower supply according to the relationship illustrated in FIG. 6.

FIG. 6 shows an embodiment of the control characteristics of oneembodiment of the method described in FIG. 5 in a pulse width modulatedcontroller where the duty ratio changes in response to the feedbacksignal. In one embodiment, a constant frequency clock defines cycles inwhich the power switch may conduct. As shown in the embodiment of FIG.6, the duty ratio D varies linearly with the magnitude of the feedbackin the region of regulated output, going between zero and the maximumD_(MAX) when the feedback signal U_(FB) is between U₂ and U₃ inaccordance with the teachings of the present invention.

In one embodiment, the regulator operates in the unregulated transitionUT region when the feedback signal U_(FB) is between U₁ and U₂. Itoperates in the auto-restart AR region when the feedback signal U_(FB)is less than U₁. In the unregulated transition region, the controllerlimits the on-time of the power switch only by a maximum duty ratioD_(MAX) or by a maximum current I_(MAX) of the power switch,substantially independent of the magnitude of the feedback signal.Ordinary pulse width modulated controllers with an auto-restart regiontypically have an unregulated transition UT region that is a negligiblysmall artifact of the design, the goal being to avoid unregulatedoperation. Embodiments of the present invention expand the unregulatedtransition region of the controller to realize a benefit in the controlof output power of a switching regulator in accordance with theteachings of the present invention.

FIG. 7 shows the control characteristics of another embodiment of themethod described in the flowchart of FIG. 5. In contrast to the pulsewidth modulated controller of FIG. 6 that uses the feedback signal tochange the duty ratio of the power of the power switch, the on/offcontroller embodiment described in FIG. 7 uses the feedback signalsimply to enable or disable the operation of the power switch. Theregion of regulated output is restricted to a narrow range determined bythe thresholds U₂ and U₃. In the simplest limiting case, U₂ and U₃ mergeto a single value U₂₃. Regulation is accomplished by variation of thefeedback above and below the threshold U₂₃ to inhibit or enable thepower switch to conduct during a clock cycle. When the magnitude of thefeedback signal U_(FB) is below the region for regulated output, thecontroller operates in either the unregulated transition UT region orthe auto-restart AR region in the same way as the pulse width modulatedcontroller illustrated in FIG. 6.

FIG. 8 shows the characteristics of the output voltage and current ofone embodiment of a switching power supply that operates in accordancewith the teaching of the present invention. Line segment 800 of theoutput characteristic is the locus of output voltage and output currentin the constant voltage CV region. Line segment 805 is the locus ofoutput voltage and output current in the unregulated transition UTregion. Line segment 810 is the locus of output voltage and outputcurrent in the auto-restart AR region. As shown, the output voltage andoutput current fall within the specified boundaries 815 and 820.

In one embodiment, a power switch, a current sensing circuit, and acontroller are combined into an integrated circuit to control the outputpower of a switching regulator. To illustrate, FIGS. 9A and 9B showschematics of embodiments of the present invention in a power supplythat uses an integrated circuit 900 to realize the on/off controlcharacteristic illustrated in FIG. 7. As shown, FIG. 9B shows exampleembodiments of some of the elements in FIG. 9A in greater detail and thefollowing discussion applies to both FIGS. 9A and 9B. An energy transferelement 907 receives a DC voltage input 908 on a primary winding 909 toprovide a DC output voltage 910 from a secondary winding 912 and a biasoutput voltage 911 from a secondary winding 913. Bias output voltage 911is coupled to output voltage 910 through the transformer action ofenergy transfer element 907 such that bias output voltage 911 is ameasure of output voltage 910. Thus output voltage 910 is regulatedindirectly by regulation of bias voltage 911.

Integrated circuit 900 is coupled to the primary winding 909 of energytransfer element 907 at a drain terminal 949. The integrated circuit 900includes a MOSFET power switch 915 and a voltage regulator circuit 947that are coupled to a drain terminal 949. Voltage regulator 947 providesa regulated voltage at a bypass terminal 945 that is coupled to a bypasscapacitor 914. The regulated voltage at the bypass terminal 945 is apower supply for the internal circuits of integrated circuit 900.

A feedback terminal 905 is coupled to resistors 906 and 916 to receive afeedback voltage that is proportional to bias voltage 911. A sourceterminal 946 is coupled to the source of the MOSFET 915.

An oscillator 971 produces a clock signal that defines the switchingcycles. The output from oscillator 971 is logic high during the time ineach cycle that corresponds to the maximum duty ratio of the powerswitch. Oscillator 971 receives a signal that reduces the frequency ofthe oscillator when the controller operates in the auto-restart region.FIG. 9B shows in detail one embodiment of oscillator 971. Currentsources 920 and 921, transistors 922 and 924, and inverter 923 provideeither high or low current to charge timing capacitor 931 according tothe frequency selection input LOWFREQ. Inverter 926, transistors 927,928, 929, 930, and non-inverting buffer with hysteresis 932 complete theoscillator circuit. In one embodiment, the frequency of the oscillatoris reduced by a factor of 20 when the frequency selection input LOWFREQis logic high.

A pulse generator circuit including inverters 936, 937, 938 and NANDgate 939 provides signals to determine the on-time of the power switch.The output of AND gate 939 is a narrow positive pulse at the beginningof each oscillator clock cycle.

The feedback voltage at the feedback terminal 905 is received bymeasurement circuit 960 that compares the magnitude of the feedbacksignal to thresholds U₁ and U₂₃ with comparators 934 and 961respectively. As long as the feedback signal is greater than U₁, theoutput of comparator 934 will be logic high to keep the output ofauto-restart counter 933 low, disabling auto-restart operation. When theregulator operates in the auto-restart region, the output of counter 933allows a fixed number of drive pulses from flip-flop 942 to reach thegate of MOSFET 915, followed by a substantially longer interval withoutdrive pulses. In one embodiment, the interval without drive pulses is 20times the interval with drive pulses. A logic high output of counter 933reduces the frequency of oscillator 971 while providing a logic lowoutput from inverter 925, to one input of AND gate 914, blocking thedrive signal from the output of flip-flop 942.

When the feedback signal U_(FB) is greater than U₂₃, comparator 961applies a logic low signal to an input of AND gate 940 that preventspulses from pulse generator 972 from setting flip-flop 942, inhibitingthe drive signal to the gate of MOSFET 915. Thus switching is inhibited.When feedback U_(FB) falls below threshold U₂₃, a logic high output ofcomparator 961 allows the output of flip-flop 942 to drive the rate ofMOSFET 915. MOSFET 915 turns off when flip-flop 942 is reset by eitherthe maximum duty ratio input or the current limit input to OR gate 941.Either event resets the output of flip-flop 942 to remove the drive fromthe gate of MOSFET 915.

Comparator 961 includes transistors 917, 918, 943, 944, with currentsources 934 and 919. Resistors 901, 902 and 903 together with currentsource 919 set the thresholds U₁ and U₂₃.

Current sense circuitry 970 includes comparator 948 that measures thevoltage on the drain of MOSFET 914 when it conducts drain current 904.The voltage on the drain is substantially proportional to the current904 by the on-resistance of the MOSFET. The output of current limitcomparator 948 is logic high when the on-voltage of the drain exceedsV_(ILIMIT). A logic high at the output of AND gate 951 will reset theflip-flop 942 to remove the drive to MOSFET 915. A leading edge blankingcircuit 950 with AND gate 951 masks the output of the current limitcomparator 948 for a short duration after MOSFET 970 turns on to avoidpremature termination of the on-time from initial high current from thedischarge of parasitic capacitance.

In the foregoing detailed description, the methods and apparatuses ofthe present invention have been described with reference to a specificexemplary embodiment thereof. It wills, however, be evident that variousmodifications and changes may be made thereto without departing from thebroader spirit and scope of the present invention. The presentspecification and figures are accordingly to be regarded as illustrativerather than restrictive.

1. A power converter controller circuit, comprising: a control circuitto be coupled to a switch, the control circuit including a current sensecircuit to be coupled to a primary winding of an energy transfer elementcoupled to the switch, the control circuit further including ameasurement circuit to be coupled to a secondary winding of the energytransfer element; a feedback circuit coupled to the control circuit andcoupled to receive a feedback-signal derived from an output quantity ofa power converter and to output a signal to the measurement circuit,wherein an output of the measurement circuit and an output of thecurrent sense circuit are coupled to control a switching of the switchto regulate an output voltage or an output current of the powerconverter, wherein the power converter control circuit controls theswitching of the switch to operate the power converter in a regulatedoutput region, a first unregulated output region and a secondunregulated output region responsive to a magnitude and duration of thefeedback signal, wherein the magnitude and duration of the feedbacksignal correspond to a combination of the output voltage and the outputcurrent, wherein the first unregulated output region is a selfprotection auto-restart region, wherein the second unregulated outputregion is an expanded unregulated transition region of the powerconverter controller circuit providing continuous output power at asubstantially maximum output power of the power converter, wherein acurrent limit of the power converter controller circuit is independentof the magnitude of the feedback signal during the second unregulatedoutput region: and an oscillator included in the control circuit toproduce a clock signal that defines switching cycles of the switch. 2.The power converter controller circuit of claim 1 wherein the powerconverter controller circuit is included in a battery charger.
 3. Thepower converter controller circuit of claim 1 wherein the controlcircuit includes an integrated circuit.
 4. The switching regulator ofclaim 1 wherein the control circuit includes discrete electricalcomponents.
 5. A method for operating a power converter, comprising:measuring a feedback signal received from a secondary winding of anenergy transfer element of the power converter; sensing a currentreceived from a primary winding of the energy transfer element, anoutput power of the power converter controlled in response to a clocksignal, the feedback signal and the sensed current; operating the powerconverter in an unregulated auto-restart mode if the feedback signal isless than a first threshold; operating the power supply in a regulatedmode if the feedback signal is between the first threshold and a secondthreshold while greater than a third threshold; and operating the powerconverter in an unregulated maximum output power mode if the feedbacksignal is between the first threshold and the third threshold, wherein acurrent limit of the power converter controller circuit is independentof the magnitude of the feedback signal during the unregulated maximumoutput power mode.
 6. The method of claim 5 further comprising operatingthe power converter in an unregulated inhibited switching mode if thefeedback signal is greater than the second threshold.
 7. The method ofclaim 5 wherein the first threshold less than the third threshold. 8.The method of claim 7 wherein the third threshold is less than or equalto the second threshold.
 9. The method of claim 5 wherein the outputpower of the power converter is nonresponsive to the feedback signalwhen the power converter is operating in any one of the unregulatedauto-restart mode, the unregulated inhibited switching mode or theunregulated maximum power mode.